For the navigational term, see Mark (navigation)
A "mark" is a slang term for the individual who is to be the target of a scam.
The Doctor was Neelix and Tom Paris's mark, for a demonstration of the shell game which was supposed to show that their deceptive skills had not weakened with time. Unfortunately, The Doctor saw through their scam due to his high visual acuity. (VOY: "Live Fast and Prosper")